The proliferation of nuclear weapons and the possible detonation of a radiological dispersal device (a dirty bomb), the deliberate damaging of a radioisotope production facility and many other terrorist scenarios have generated a requirement for detecting nuclear materials from a distance. Nuclear materials are difficult to detect with most presently available detectors such as gamma ray detectors, because such detectors must be close to a nuclear source, i.e. within the radiation field generated by a radioactive source in order for detection to occur.
Nuclear materials generate ionizing radiation in the surrounding atmosphere, which creates air luminescence that can be imaged from a distance. Air luminescence is a result of de-excitation of excited molecular nitrogen and nitrogen molecular ions (N2+) created by the ionizing process. The wavelength of the luminescence lies primarily in several ultraviolet bands between 280 and 425 nm and has unique spectral fingerprints consisting primarily of N2 second positive group emissions.
U.S. Pat. No. 6,448,562, issued to William A. Seidler et al mentions this principle in order to provide a remote sensor for detecting a nuclear source by obtaining an image of the source from photons generated by ionizing radiation in ultraviolet spectral regions. The patent teaches the use of an optical system for remotely detecting selected wavelengths of photon emissions from an airglow caused by a source of ionizing radiation by filtering sufficient out-of-band wavelengths while transmitting enough in-band wavelengths to allow detection of nuclear material with the selected wavelengths. The invention may be used to remotely detect N2+ line emissions or other ultraviolet (UV) line emissions having defined naturally occurring properties. The camera used in the invention responds to photons resulting from “airglow” caused by gamma rays emitted from a nuclear material source which hit atmospheric N2 and result in N2+ ions. The N2+ ions then emit photons at wavelengths of, for example 391.4 nm and 358.4 nm, as they return to a groundstate. The patent teaches that the nitrogen molecule in an excited state emits ultraviolet photons with a wavelength of 391.4 nm. (and others, for example 337.1 nm, 357.7 nm and 380.5 nm).
The patent also teaches that naturally occurring “airglow” in the upper atmosphere and in parts of the “ionosphere” normally found about 40-100 kilometers (km) above the Earth's surface but varying around the Earth results from naturally occurring electrons, gamma rays, and X-rays from space which bombard N2 atoms in the ionosphere producing N2+ ions and photons, and that natural “airglow” occurs from solar winds colliding with the ionosphere. However, at about 20 kilometers altitude, virtually no naturally occurring gamma rays or X-rays are present and that the 391.4 nm wavelength was selected for the operating wavelength due to the fact that at night the UV region of the spectrum has very low background noise below 20 km altitude.
However, there is background light in the UV region of the spectrum at night caused by reflected moonlight or street lamps at night.
The air luminescence induced by ionizing radiation from nuclear material is typically 108 (i.e. 100 million) times weaker than daylight, which necessitates measurements during darkness.